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Override the Type of a JSON File

Matt Pocock
Matt PocockMatt is a well-regarded TypeScript expert known for his ability to demystify complex TypeScript concepts.

Need to manually type a JSON file?

I learned today that you can add a .d.json.ts file to manually assign a type to a JSON import.

For example, if you have a data.json file, you can create a data.d.json.ts file and define the type inside:

// data.d.json.ts
declare const data: Record<string, string>;

export default data;

You can change the Record<string, string> type to match the desired shape.

To make TypeScript recognize .d.json.ts files, you need to add a setting in your tsconfig.json:

"compilerOptions": {
"allowArbitraryExtensions": true

This setting lets TypeScript use .d.json.ts files.

Why Would You Do This?

This technique can be particularly useful when working with large .json files used for test fixtures. If TypeScript infers the type of all the data in the file, it can significantly slow down the TypeScript transpilation process - and your IDE.

By assigning a broader type like Record<string, string>, TypeScript will use the specified type instead. This optimization can greatly improve the performance of your TypeScript transpilation.

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